It's one of the most controversial ideas in archaeology. Humans from Europe supposedly could have crossed the frozen Atlantic and reached America first. It's a fringe idea...but now two archaeologists have released a book explaining how it could have happened.

In their new book Across Atlantic Ice, University of Exeter archaeologist Bruce Bradley and Smithsonian Institution colleague Dennis Stanford outline how they think early humans could have made it from Europe to the Americas between 18,000 and 25,000 years ago. The pair first proposed their controversial hypothesis thirteen years ago, but their new book attempts to solve one of the hypothesis's biggest problems: just how could humans have crossed the Atlantic?

Earlier versions of this hypothesis had required Bradley and Stanford to suggest the presence of advanced seafaring technology among these ancient humans that was on par with what the Polynesians used to colonize the Pacific. Their new idea is rather less far-fetched, suggesting that climate conditions during the time period created a sort of icy highway between France and North America. The authors say these ancient humans, known as the Solutrean culture, could have made it across the Atlantic use technology known to be in their possession.

Basically, they could have crossed the Atlantic using technology roughly along the same lines of how native Alaskans navigate the Arctic. Using small boats, the Solutreans could have hugged the coast of the pack ice, melting iceberg ice for freshwater and catching fish and seals for food and fuel. Bradley and Stanford point to the discovery of a 20,000 year old toolkit of bone needles that very closely resembles those used by Eskimos today - so much so that in a recent BBC program an Eskimo woman identified the artifact as belonging to her culture.

The major evidence driving Bradley and Stanford's hypothesis is the presence of artifacts found in six sites in the eastern United States dating to somewhere between 18,000 and 26,000 years ago. These tools more closely resemble that of the Solutrean culture that endured in Europe between about 21,000 and 17,000 years ago than the Clovis culture that first appeared in North America about 13,000 years ago.

Bradley and Stanford propose that a relatively small subset of the larger Solutrean technology made it all the way across the Altantic ice, and they were responsible for these ancient tools in the eastern United States. As a much larger wave of humans crossed into North America from Asia a few thousand years later, the Solutreans were either wiped out or absorbed, and their tool technology was absorbed into that of Clovis.

It's certainly an intriguing idea, and the authors can point to some compelling points of evidence, but that's really what they are: small points awash in a sea of contradictory evidence. While the sea ice theory is possible - certainly far more possible than imagining Solutreans somehow crossing the open Atlantic - there's very little evidence on either side of the Atlantic of any maritime culture from that time. Besides, it's far from a certainty that the pack ice so crucial to Bradley and Stanford's hypothesis actually existed, at least to the necessary extent to allow for a transatlantic crossing.

There's also genetic evidence to consider. Two separate studies of mitochondrial DNA undertaken by Brazilian researchers in 2008 and Italian researchers in 2011 provide no evidence for a second origin point for the American population, instead strongly suggesting that the ancestors of indigenous Americans only entered the New World from Asia.

There are also plenty of alternative ways to interpret the archaeological evidence. Yes, there seem to be artifacts in the eastern United States that significantly predate any found around the Bering Strait, but that's hardly evidence that there are no similarly old artifacts waiting to be discovered in Alaska. The resemblance between these American artifacts and those of the Solutrean culture are striking, but far from conclusive - it's certainly possible that it's all just a coincidence, particularly when you consider just how unlikely the story is that would explain how the Solutreans reached the Americas.

At the end of the day, the Solutrean hypothesis is a neat idea, even though it does make me a bit uneasy how it seems to recall old narratives about advanced ancient Europeans reaching the Americas before anyone else and leaving behind super technology - I'd say that resemblance is coincidental, but it is why I've pointedly avoided calling the Solturean culture "European" throughout this post, preferring "humans from Europe."

For all the intriguing evidence Bradley and Stanford have been able to marshal, this still seems like a case where they are invoking a massive change to known history to solve a relatively minor problem. Our current dating of really ancient American artifacts is still far from perfect, but it does seem we have good evidence of pre-Clovis tools. But right now, it seems a hell of a lot easier to say humans simply crossed over from Asia earlier than we thought they did than to presuppose an entire transatlantic trek.